Tides as a Major Influencing Factor to Shoreline Debris Deposition

Scott Wilson

Centre for Environmental Management

Shorelines as sinks for marine debris

• Most commonly studied areas for marine debris– Accessibility– Visibility– Directly effect most people

• Surveys and coastal cleanups target these areas

• Most however failed to understand the factor influencing the why and the how

• What causes debris to end up on our shorelines?

Factors that influence coastal debris movement and deposition

• Nearness to source

• Type of debris (eg. durability)

• Form of debris (eg. buoyancy, shape)

• Physico-chemical parameters

• Coastal geomorphology

• Wind

• Currents

• Rainfall and storm events

• TIDES

Ocean Tides

• Rise and fall on a daily basis

• Moon and Sun’s gravitational forces along with the earth rotation influences tidal patterns

• Tides can be classed as diurnal, semi diurnal and mixed

• Shape of the coastline and bathymetry cause localised patterns

Shoreline Morphology

• Wave dominated– Low - Moderate energy

– Steep slope

• Intermediate– High Energy

– Bars

• Tide-modified– Low energy

– High tidal range

• Tide-dominated– V. Low energy

– Gentle slope

Beach Debris Amounts Over Time

20-O

ct-08

27-O

ct-08

03-N

ov-0

8

10-N

ov-0

8

17-N

ov-0

8

24-N

ov-0

8

01-D

ec-0

8

08-D

ec-0

8

Deb

ris (

no.)

0

100

200

300

400

500

600

700

Tid

al H

eigh

t (m

)

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6CanoeLilliesTide

Tides and Debris

• Temporal debris collection shown to have a strong correlation with tidal height

R2 0.69 & 0.62 (P<0.05)

Tides and Debris

• Not all beaches reflect this strong trend

R2 = 0.36 (p>0.05)

Tides and Debris

• Analysis of within beach zones draws further trends

R2 0.5

R2 0.4

R2 0.1

Tides and Debris

• Debris type helps explain the differences

Considerations for maximising outcomes

• Debris surveys and Clean-ups should consider site characteristics, weather and tidal patterns

• Potential to model and predict patterns of accumulation

• Maximum debris deposition after full and new moons

• Timing is everything!• Other factors do play a part